Discrete fillers include functionalized nanotubes, discrete carbon fibers, multi-walled nanotubes, ceramic powders, metallic nanoparticles, silicates, or single-walled carbon nanotubes. Such fillers have unique mechanical, thermal, and electrical properties that would be useful in industrial applications. However, in order to be utilized in most applications, the discrete fillers must be placed in a matrix of some type to form a composite.
Certain composites have been disclosed. For example, solvent casting followed by melt mixing has been used to produce single-walled carbon nanotube/poly(methyl methacrylate) composite fibers with enhanced elastic modulus and electrical conductivity (Haggenmueller, R.; Commans, H. H.; Rinzler, A. G.; Fischer, J. E.; Winey, K. I. Chem. Phys. Lett. 2000, 330, 219). Single-walled carbon nanotube/polystyrene composite films with high electrical conductivity have been made using a miniemulsion polymerization method (Barraza H. J.; Pompano, F.; Orear, E. A.; Resasco, D. E. Nano Letters 2002, 2, 797). Likewise, multi-walled carbon nanotube/poly(propylene) composites made by shear mixing method showed a significant increase in decomposition temperature due to presence of the nanotubes (Kashiwagi, T.; Grulke, E.; Hilding, J.; Harris, R.; Awad, W.; Douglas, J. Macromol. Rapid Commun. 2002, 23, 761). However, such composites have not yet demonstrated properties approaching their theoretical potential. One of the biggest challenges faced in this area is to obtain a uniform dispersion of fillers in a polymer matrix.
Adequate dispersion is a key factor in composite performance, and discrete fillers are prone to aggregation. For example, single-walled carbon nanotubes (“SWNTs”) are fullerenes in the shape of a hollow tube, usually with a diameter from 0.7-2 nm. SWNTs can be formed by a variety of conventional methods, some of which are described in U.S. Pat. No. 6,544,463, the disclosure of which is incorporated herein by reference in its entirety. SWNTs possess a unique combination of strength, high modulus of elasticity, and excellent heat and electrical conductivity, but SWNTs are prone to aggregation (“bundling”) due to Van der Waals attraction among the tubes, which have large surface areas. Many previous attempts to uniformly disperse SWNTs throughout a polymer matrix have been less than fully successful.
It has now been discovered that discrete fillers, including SWNTs, may be uniformly dispersed throughout a polymer matrix to form a composite using methods of the present invention.